To achieve high-speed printing, an electrophotographic color image forming apparatus is known to include independent image forming units for respective colors, sequentially transfer images from the image forming units for respective colors onto an intermediate transfer belt, and collectively transfer images from the intermediate transfer belt onto a recording medium.
Each of the image forming units for respective colors includes a photosensitive drum as an image bearing member. Each image forming unit further includes a charging member for charging the photosensitive drum and a developing unit for developing a toner image on the photosensitive drum. The charging member of each image forming unit contacts the photosensitive drum with a predetermined pressure contact force to uniformly charge the surface of the photosensitive drum at a predetermined polarity and potential by using a charging voltage applied from a voltage power supply dedicated for charging (not illustrated).
The developing unit of each image forming unit applies toner to an electrostatic latent image formed on the photosensitive drum to develop a toner image (visible image).
In each image forming unit, a primary transfer roller (primary transfer member) facing the photosensitive drum via the intermediate transfer belt primarily transfers the developed toner image from the photosensitive drum onto the intermediate transfer belt. The primary transfer roller is connected to a voltage power supply dedicated for primary transfer.
A secondary transfer member secondarily transfers the primarily transferred toner image from the intermediate transfer belt onto a transfer material. A secondary transfer roller (secondary transfer member) is connected to a voltage power supply dedicated for secondary transfer.
Japanese Patent Application Laid-Open No. 2003-35986 discusses a configuration with which each of four primary transfer rollers is connected to each of four voltage power supplies dedicated for primary transfer. Japanese Patent Application Laid-Open No. 2001-125338 discusses control for changing, before image formation operation, a transfer voltage to be applied to each primary transfer roller depending on sheet-passing durability of an intermediate transfer belt and a primary transfer roller and on resistance variation due to environmental variation.
However, a conventionally known image forming apparatus using an intermediate transfer belt has the following problem.
With the primary transfer configuration with which toner images are reverse transferred from a plurality of photosensitive drums arranged on a line onto the intermediate transfer belt, the polarity of the transferred toner images on the intermediate transfer belt may be inverted by electric discharge occurring at the primary transfer sections formed by the intermediate transfer belt and the photosensitive drums. In this case, toner having the inverted polarity may move from the intermediate transfer belt to the photosensitive drums. This phenomenon is referred to as reverse transfer.
Specifically, during printing a full color image, a transferred toner image of the first color on the intermediate transfer belt is electrostatically attracted to the intermediate transfer belt. However, when the toner image of the first color passes through gaps between the intermediate transfer belt and the photosensitive drums of the second and subsequent colors, the toner image of the first color may partially be reverse transferred onto the photosensitive drums. Reverse transfer will cause image unevenness, density reduction, or other problems. The potential difference at the primary transfer sections can be reduced by decreasing the voltage applied to the primary transfer members, thus preventing reverse transfer. However, there is a tendency that decreasing the potential difference at the primary transfer sections decreases the primary transfer efficiency, thus making it difficult to set a primary transfer voltage that can be set to prevent reverse transfer.